There are a few major categories of foundation pile: driven pile, bored pile, injected pile. etc. This invention concerns only the first category, i.e. the driven pile. In the foundation construction industry, the most important aspect of a pile to a foundation engineer is the load-bearing capacity of the pile, i.e. how much load the pile may bear with respect to the building or structure to be built on the foundation.
Foundation engineers are engaged to design and choose pile types as laid down by the design requirements or standards (such as the British Standard BS 8004:1986). The foundation design is normally made after taking into consideration the columnar transferred load weight, soil type and conditions, piling system and pile design. Since soil type is an existing condition and the columnar transferred load has been predetermined in the design, the engineer may only advise on the two remaining variable factors, i.e. (i) pile design and (ii) piling system. In the present invention the piling system is the pile driving system.
For example, if the proposed pile is calculated to impose an estimated load of 300 tonnes via a structural column above the pile, the foundation engineer may assign a safety factor of 2 (SF=2, or twice the load on the pile, depending on the method of calculation), thus 2.times.300 tonnes=600 tonnes is the load the pile to be designed or chosen must be able to bear or support.
Foundation engineers then conduct tests on actual piles at a site based on that pile design. The piles are driven into the soil by hammer blows. Certain methods of calculations such as the Dynamic Pile Formulae, are used to predict the load-bearing capacity of the pile for a certain depth of penetration.
The prototype pile may then be statically and/or dynamically tested at-site to ascertain its load-bearing behaviour. This is a costly and time-consuming process.
As in the above example, if the field test shows that the (soil) resistance, R, is less than 600 tonnes, the pile design is considered to have failed and a new pile design is then sought and the process is repeated. The conditions at site, equipment used and current methods further make it impossible for an accurate prediction or measurement that is acceptable to a design requirement.
Dynamic Pile Formulae. For centuries, engineers have relied upon the number of hammer blows per unit of pile penetration to estimate the load-bearing capacity of the driven pile. Engineers have equated the hammer energy to the work done advancing the pile against the soil resistance.
There are many formulas used to determine the capacity of the piles driven into the soil. Theoretical and semi-empirical formulas are derived to express this relationship between energy and work. These equations are generally known as Dynamic Pile Formulae which is derived from Newton's Second Law of Motion and which are the most widely accepted formulas to determine the load-bearing capacity for driven piles.
The model common to all the simple Dynamic Pile Formulas is illustrated in FIG. 1 as ##EQU1##
This basic dynamic pile-capacity formula, which is also termed rational pile formula, depends on impulse-momentum principles and nearly all the dynamic pile formulas currently in used are based on this equation. [See, for example, Joseph E. Bowles (1988), Foundation Analysis and Design, 4th ed., McGraw-Hill, p. 791].
Some of the specific formulas derived from the basic formula, in order to improve the reliability of predictions on the pile capacity by making various assumptions, each giving a different value of pile capacity, are given in the following table [Bowles, p.
TABLE I ______________________________________ Various Dynamic Pile formulas ______________________________________ Danish formula [Olson and Flaate (1967)] (use SF = 3 to 6) ##STR1## Eytelwein formula (use SF = 6) [Chellis (1961)] ##STR2## Modified ENR (Engineering News-Record (1965)] (use SF = 6) ##STR3## Hiley Formula (1930) ##STR4## ______________________________________ wherein P.sub.u = ultimate pile capacity, F. A = pile crosssection area, L.sup.2. E = modulus of elasticity, FL.sup.-2. e.sub.h = hammer efficiency. E.sub.h = manufacturers' hammerenergy rating, FL. h = height of all of ram, L. k.sub.1 = elastic compression of capblock and pile cap and is a form of P.sub.u L/AE, L. k.sub.2 = elastic compression of pile and is of a form of P.sub.u L/AE, L k.sub.3 = elastic compression of soil, also termed quake for waveequation analysis, L. L = pile length, L. n = coefficient of restitution. s = amount of point penetration per blow, L. W.sub.p = weight of pile including weight of pile cap, driving shoe, and cap block (also includes anvil for doubleacting steam hammers), F. W.sub.r = weight of ram (for doubleacting hammers include weight of casing), F.
Each of the formulas has their own respective advantages. For example, the modified ENR formula is thought to be reasonably valid over the entire range of load test. The Hiley formula is found to be with the least statistical deviation or highest statistical correlation [Bowles, p. 802]. The British Standard, BS 8004:1986, cited the Hiley formula as one of the more reliable and is probably the most commonly used in Britain.
Although dynamic formulas have been widely used to predict pile capacity, more accurate means is needed to determine when a pile has reached a satisfactory load bearing value other than by simply driving it to some depth predetermined by the formulas. This is because driving the pile to a predetermined depth may or may not obtain the required bearing value due to normal soil variation both laterally and vertically. [Bowles, ibid.] It is generally accepted that the dynamic formulas do not provide very reliable predictions but are continued to be used for lack of at better method.
There is therefore a need for a better method for accurately predicting a pile's load-bearing capacity. Due to the harsh external environment at foundation construction sites, it would be desirable if the pile capacity instrumentation and measurements could be conducted within an indoors facility such as in a geotechnical laboratory. The physical dimension limitations of an indoors facility means that the actual dimensions of the pile is preferably be scaled-down so that the pile and its requisite instrumentation or measurement means may be accommodated within the indoors facility.
Such a proposed scaled-down pile model will need a mathematical relationship in order to correlate its values to the pile's values so that the model may be used as an industrial application in estimating the capacity of a pile.
In this specification, the meaning of the words "prototype pile", "actual pile" and "pile" have been used interchangeably in view of the same physical characteristics of the piles when correlated to a model pile described herein.